1. Field of the Invention
The invention relates to the field of wavelength lockers used in conjunction with widely tunable lasers, and more particularly, to a partially mirrored beam tap for wavelength lockers.
2. Description of the Related Art
There are a variety of semiconductor laser designs in which the wavelength can be tuned by current injection into one or more sections of the laser that have some spectral filtering characteristics. Examples include moderately-tunable devices, such as distributed Bragg reflector (DBR) lasers, and widely-tunable devices, such as sampled grating distributed Bragg reflector (SGDBR) lasers.
In each of these devices, precise control of each of the currents provided to the various sections of the laser is required to achieve a desired output wavelength. The amount of current injected into each section to achieve a given wavelength can vary with both device temperature and aging. For example, over time, as the device ages, changes in leakage current and carrier lifetime in the tuning sections cause the laser output wavelength to drift. This variation in tuning characteristics requires the use of some sort of feedback control system in order for such tunable lasers to be used in applications where precise wavelength control must be maintained, such as dense wavelength division multiplexed (DWDM) communications systems.
One type of feedback control is known as a wavelength locker. A wavelength locker is used for stabilizing the laser output wavelength at one of a plurality of channels. However, prior art wavelength lockers greatly reduce optical output, because they tap too large a portion of the light for wavelength locking, thereby reducing fiber coupling efficiencies. Moreover, prior art wavelength lockers are not especially compact, and thus are not easily integrated into laser device packaging. In addition, prior art wavelength lockers have not been cost effective.
Therefore, there is a need in the art for a wavelength locker that does not reduce optical output as much, so that fiber coupling efficiencies are not as reduced. In addition, there is a need in the art for a wavelength locker that is more easily integrated into laser packaging. Moreover, there is a need in the art for a wavelength locker that is cost effective.